Epicardial and endocardial electrodes have been used to record S-T segment elevation in the area of an acute myocardial infarction in open chest dogs, with careful control of hemodynamic variables. Five minute occlusions of the distal left anterior ascending coronary artery under constant hemodynamic conditions caused reproducible epicardial and endocardial S-T segment elevation. The endocardial and epicardial S-T segment elevation caused by a five minute coronary occlusion is increased by increasing cardiac output, increasing heart rate or decreasing aortic pressure when other hemodynamic variables remain constant (p less than .05). Elevation of aortic pressure caused a greater decrease in epicardial S-T elevation than in endocardial S-T elevation (p less than .05) while changes in cardiac output or heart rate caused alterations in epicardial S-T elevation which were quantitatively similar to changes in endocardial elevation. Nitroglycerin infusion in these animals, at a constant heart rate, aortic pressure, and cardiac output caused no change in endo-cardial or epicardial S-T segment elevation with a five minute coronary occlusion.